1. Field of the Invention
The present invention relates to a power transfer system including a power transmission unit and a power reception unit.
2. Description of the Related Art
As the size, weight, and power consumption of electronic devices decrease and as the capacity of batteries increases, more and more battery-driven electronic devices have been used recently. Also, wireless data communication has been increasingly used between devices at a close distance. In light of such circumstances, a technique for transferring power in a non-contact manner is demanded.
For example, as presented in Japanese Unexamined Patent Application Publication No. 2008-206327, existing non-contact charging systems include a power transmission unit which includes a primary coil provided in a charger cradle or the like, and a mobile electronic device which includes a secondary coil and a rechargeable battery. The user places the mobile electronic device on the power transmission unit. This consequently generates electromagnetic inductive coupling (magnetic coupling) between the primary coil included in the power transmission unit and the secondary coil included in the mobile electronic device. As a result, power is supplied to the to-be-charged device side and the secondary battery is charged.
However, in the power transfer system of Japanese Unexamined Patent Application Publication No. 2008-206327, the power transmission coil and the power reception coil function as an electromagnetic-induction-based isolation transformer and thus are used merely as a magnetic-coupling-based transformer. It is essential for electromagnetic-induction-based transformers to efficiently convert electric energy into magnetic energy and then into electric energy by linking magnetic flux produced by current flowing through the primary winding to the secondary winding so as to cause current to flow through the secondary winding. A ratio of magnetic flux linked to the secondary winding to magnetic flux produced by current flowing through the primary winding is generally called the amount of (magnetic) coupling. In order to increase the power conversion efficiency, it is essential for electromagnetic-induction-based transformers to increase the degree of magnetic coupling. However, in many cases, it is difficult to increase the degree of magnetic coupling in transformers in order to prevent magnetic saturation or because of physical restraints. As a result, the power conversion efficiency decreases.
In addition, in power transfer systems, because power is transferred after impedance matching (matching) is performed, the operating frequency is generally changed during control. In electronic devices, on the other hand, the usable frequency band is decided for each device. Thus, operation at a fixed frequency is preferable in the light of EMC (electromagnetic compatibility), transferred energy controllability, and so forth.